Methods for Realizing Power Balance in an

Interleaved Configuration of LLC Resonant

Converters

Heiko Figge, 13.10.2012, IBM Technology Symposium

IBM Technology Symposium

13.11.2012

Why going for LLC ?

Why going for interleaved LLC ?

The balancing issue of interleaved LLC

Solutions to the balancing issue

Measurement results

2

Outline

IBM Technology Symposium

13.11.2012

LLC topic addresses the PSU DC-DC converter stage

3

Why going for LLC ?

Server

Loads

VR

VR

Fans

AC/DC

PSU

DC-DC

Rack

Server

1ɸ AC in

IBM Technology Symposium

13.11.2012

80plus Efficiency levels for 12V output units (high line)

4

Why going for LLC ?

80

84

88

92

96

0 10 20 30 40 50 60 70 80 90 100 110

Titanium

Platinum

Gold

Silver

Bronze

Load in %

Effic

iency in %

IBM Technology Symposium

13.11.2012

Titanium efficiency-level goal (96% peak efficiency)

LLC has demonstrated

best-in-class efficiency in case of 54V 2-3kW output

(96%+ rectifiers)

best-in-class efficiency in case of 12V <1kW output

(96% peak efficiency rectifiers)

high robustness against abnormal operating conditions, e.g.

short-circuit

capability of linear ramp-up

5

Why going for LLC ?

IBM Technology Symposium

13.11.2012

Why going for LLC ?

Why going for interleaved LLC ?

The balancing issue of interleaved LLC

Solutions to the balancing issue

Measurement results

6

Outline

IBM Technology Symposium

13.11.2012

+

Lm

CsS1

S2

Vbulk

Cout

D2

D1

Vout

iriout

im

n:1:1Ls

RL

Iout

Principal schematic of LLC DC-DC converter

7

Why going for interleaved LLC ?

outoutCout Iii

Kirchhoff's current law

IBM Technology Symposium

13.11.2012

8

Why going for interleaved LLC ?

outI

outI

)(tio

)(_ ti Ao

t

t)(_ ti Bo

Couti

not interleaved

interleaved

IBM Technology Symposium

13.11.2012 9

Why going for interleaved LLC ?

Example: Vout = 12V and Pout = 3000W

Iout = 250A

RMS ripple current taken by output capacitor Cout

not interleaved: ICout = 154A rms

interleaved: ICout = 31A rms

Interleaving reduces output current ripple by factor 5

IBM Technology Symposium

13.11.2012

Why going for LLC ?

Why going for interleaved LLC ?

The balancing issue of interleaved LLC

Solutions to the balancing issue

Measurement results

10

Outline

IBM Technology Symposium

13.11.2012

Interleaved LLC in a typical front-end structure

11

The balancing issue of interleaved LLC

1Ф AC in DC link DC out

LLC

Rail A

Vout

Iout

LLC

Rail B

iout_A

iout_B

CoutCbulkVbulk

PFC

Stage

IBM Technology Symposium

13.11.2012

Interleaved LLC may result in unbalanced power distribution

12

The balancing issue of interleaved LLC

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00%

20%

40%

60%

80%

100% LLC has passive

components Ls,

Cs, Lm

assumed

tolerance here:

±2%

graph based on

worst-case

assumption

normalized switching frequency (fs/f0)

typical

operating

area

graph data based on

time-domain simulation

sw. frequency of

both LLC rails is

pinned together

power per LLC rail / %

Rail A

Rail B

IBM Technology Symposium

13.11.2012

Why going for LLC ?

Why going for interleaved LLC ?

The balancing issue of interleaved LLC

Solutions to the balancing issue

Measurement results

13

Outline

IBM Technology Symposium

13.11.2012

Power balancing by input voltage control

14

Solutions to the balancing issue

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0300

350

400

450

500

normalized switching frequency (fs/f0)

bulk voltages / V

ΔVbulk ≈ 13V

graph shows

balanced

condition

assumed

tolerance here

±5%

graph based on

worst-case

assumption

graph data based on

time-domain simulation

Rail A

Rail B

IBM Technology Symposium

13.11.2012

Interleaved LLC in a “Split-Bulk” structure

15

Solutions to the balancing issue

LLC

Rail A

Vout

Iout

LLC

Rail B

iout_A

iout_B

Cout

Cbulk_B

Cbulk_A

Vbulk_B

Vbulk_APFC

Rail A

PFC

Rail B

1Ф AC in DC link DC out

IBM Technology Symposium

13.11.2012

Interleaved LLC in a “Two-Bulk” structure

16

Solutions to the balancing issue

3Ф AC in DC link DC out

LLC

Rail A

Vout

Iout

LLC

Rail B

iout_A

iout_B

Cout

Cbulk_B

Cbulk_A

Vbulk_B

Vbulk_APFC

Stage

(e.g.

Vienna-

Rectifier)

IBM Technology Symposium

13.11.2012

Why going for LLC ?

Why going for interleaved LLC ?

The balancing issue of interleaved LLC

Solutions to the balancing issue

Measurement results

17

Outline

IBM Technology Symposium

13.11.2012 18

Measurement Results

Efficiency status of current P1 lab unit (incl. Bridgeless PFC)

84

88

92

96

0 10 20 30 40 50 60 70 80 90 100 110

Eff[

%]

Load [%]

P1 status

Titanium

12V, 3000W

1.6 kW/l (26W/in3)

Full digital control

230V AC

IBM Technology Symposium

13.11.2012 19

Measurement Results

Waveforms measured on current P1 lab unit

100% load (Iout = 245A)

~Vbulk_A (20V/div)

~Vbulk_B (20V/div)

ir_A (10A/div)

ir_B (10A/div)

t (4ms/div)

IBM Technology Symposium

13.11.2012 20

Measurement Results

Waveforms measured on current P1 lab unit

30% load (Iout = 73A)

~Vbulk_A (20V/div)

~Vbulk_B (20V/div)

ir_A (10A/div)

ir_B (10A/div)

t (4ms/div)

IBM Technology Symposium

13.11.2012 21

Summary

LLC resonant converter enables Titanium-Efficiency designs

High-Power units benefit from interleaving technology

In case of LLC, interleaving reduces the current ripple flowing

in the output filter capacitor by factor 5

Power Balancing by the separate bulk voltages control method

overcomes the balancing issue in a quite reliable way

Complete DVT of P1 lab unit is in process

IBM Technology Symposium

13.11.2012

www.deltaenergysystems.com

Smarter. Greener. Together.

Heiko Figge

heiko.figge@delta-es.com

+49 2921 987 451

Coesterweg 45

Soest, Germany